1. Field of the Invention
The present invention relates to a lifting device, and more particularly, to a lifting device having two cylinders rigidly fastened to a horizontally movable structural component part from which load carrying means are suspended.
2. Description of the Related Art
The lifting device disclosed in German Patent Application DE 42 19 370 A1 has a movable structural component part arranged as a traveling crane trolley which is movable on a bridge or overhead traveling crane and as a part of a boom or gantry of a crane vehicle. The lifting device includes load carrying means, especially grip frames or spreaders for containers, semitrailers and the like. In known lifting mechanisms, the load carrying means are securely connected with the load to prevent relative movement therebetween during loading and unloading.
When loading and unloading containers, interchangeable superstructures or the like, the latter are lifted and lowered vertically as well as moved horizontally. However, the short movement periods necessary to optimize the processing capacity of the lifting device disadvantageously produce large accelerations and correspondingly large transverse forces.
It is recognized that pendulum-like swinging motion occurs in load carrying means suspended by cables due to the traveling motion of the horizontally movable structural component parts. This causes problems particularly when the load must be set down in precise fashion, e.g., on the frame of a vehicle. The same problem occurs as the result of wind forces. The swinging motion must be compensated for by costly cable guides or rocking inhibitors.
To solve the problem of swinging, the lifting device disclosed in DE 42 19 370 A1 proposes the use of piston-cylinder units between the horizontally movable structural component part and the load carrying means in place of the cables used previously. At least six such piston-cylinder units are supported between the movable structural component part and the load carrying means via a traveling articulated arrangement. At least four of the six or more control cylinders are inclined and fixed in place to compensate for the unwanted swinging movement.
However, this solution is very cumbersome and requires synchronization of the piston-cylinder units when raising and lowering the load--which is difficult to produce in cylinders articulated on various spatial axes. Moreover, the use of six or more spatially arranged lifting cylinders is costly in terms of procurement and maintenance.